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This talk activated the inventy bits of my brain.
Consider a simple propulsion unit for a boat.
One moving part. Runs on any fuel.
Water in, heat to steam, blow out the back for thrust.
Essentially a jet engine using water.

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At standard conditions, steam at 213 degrees F would be superheated, by 1 degree.

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And would be completely worthless for motive force!

Since we have invested so much energy creating a compressed gas out of a liquid we may as well increase the heat energy of that gas so we can get as much work as possible from it before we have to do it all over again. This increases the efficiency of the system. So it can be seen that the term superheat virtually always means high temps.

Unsuitable oils as well as incorrect delivery and dosage will lead problems such as carbon deposits, etc much quicker than saturated systems which have the issue of washing out the oil.

I remember reading a modeling thread of a man that built a superheated system for his tethered hydroplane and he said the pipe coming out of the boiler to the engine glowed dull red so he had to disassemble the engine every couple of runs to clean and oil it. Now thats some serious heat!

Well-Known Member

Steam can be done. There are new supercritical steam turbines that vastly reduce turbine size.

Basically it’s steam at high pressure that behaves like a liquid. Instead of expanding it out to low pressure with multiple blades the waste steam is returned to be reused like a heat exchanger.

Let’s say the steam is 700psi. The steam is about times as dense as a 50:1 modern jet turbine. Very thick. The thick high pressure steam is expanded until it hits maybe 500psi, then slightly cooled. At this high pressure the steam condensed to liquid at around 500 degrees as well.

That liquid can then be pumped back into the high pressure boiler using about 1/15th the energy output, where the temp is increased again. So there is next to no loss and the only energy required is raising the temp back up between cycles.

This basically just shrinks the size down where you can use a single stage turbine like a liquid.

And those are just examples of pressures, in reality they are far higher. But you get the theory.

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Note that I said that saturated steam is possible up to 705 F. It takes knowing the pressure and temperature to determine the steam condition.

Recall that at standard conditions, H2O is liquid water. Add energy (heat) to it, and it’s temperature rises. At the boiling point, 212 F, it starts to change state from water to steam. When some of the molecules have become energetic enough to be steam, but not all molecules have, the fluid is a mixture of water and steam. Increasing the heat input rate increases the speed of changing the water to steam, but does not increase the temperature. When all of the molecules have become energetic enough to be steam, the fluid is called saturated steam. Additional heat will raise the energy level and the temperature. Steam that is hotter than the saturation temperature is called superheated steam.

I did the numbers some years back. The trick to a lightweight boiler is to minimise wall thickness. The pressure that a tube of gien wall can take is inversely proportional to it's diameter. So for a lightweight boiler, use small diameter tubing in a flash boiler. You'll find that if you use 3/16 cupronickel brake line, you can build a surprisingly light boiler. Cupronickel is used in industrial boilers, it just so happens that we can buy cheap coils of it. Throw much more money into smaller or higher strength tubing, you can go even lighter. I was looking at something I could afford...

Well-Known Member

I did the numbers some years back. The trick to a lightweight boiler is to minimise wall thickness. The pressure that a tube of gien wall can take is inversely proportional to it's diameter. So for a lightweight boiler, use small diameter tubing in a flash boiler. You'll find that if you use 3/16 cupronickel brake line, you can build a surprisingly light boiler. Cupronickel is used in industrial boilers, it just so happens that we can buy cheap coils of it. Throw much more money into smaller or higher strength tubing, you can go even lighter. I was looking at something I could afford...

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So do you think a flash tube boiler 100% loss engine would be lighter than an eternal heat generated super heated water tank (read plug in the tank heater on the ground until up to temperature/pressure)

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I would appreciate a reference; the supercritical steam cycle equipment that I operated, tested and maintained have been decommissioned, so they definitely can not be considered as new.

Thanks,

BJC

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Oh this explains a lot, you have not been reading past the first few lines before jumping to conclusions.

If you had, you would have seen the part about the difference being in the condenser and not the pressure. Not having a vacuum condenser and instead returning steam while it’s still at extreme temperatures to keep fluid density high.

Well-Known Member

I did the numbers some years back. The trick to a lightweight boiler is to minimise wall thickness. The pressure that a tube of gien wall can take is inversely proportional to it's diameter. So for a lightweight boiler, use small diameter tubing in a flash boiler. You'll find that if you use 3/16 cupronickel brake line, you can build a surprisingly light boiler. Cupronickel is used in industrial boilers, it just so happens that we can buy cheap coils of it. Throw much more money into smaller or higher strength tubing, you can go even lighter. I was looking at something I could afford...

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Even those aren’t too bad. Most of the weight is the heat exchanger pipes, as they did not have efficient heat exchangers like today. The boiler on a 6000hp locomotive is about the size of a man.

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Oh this explains a lot, you have not been reading past the first few lines before jumping to conclusions.

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No jumping to conclusions, just asking for reference. You wrote

There are new supercritical steam turbines that vastly reduce turbine size.

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Since I have experience with the first generation of utility scale supercritical cycles, I would like to get current on the “new supercritical steam turbines that vastly reduce turbine size.” Why not provide an answer rather than a glib comment?

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So do you think a flash tube boiler 100% loss engine would be lighter than an eternal heat generated super heated water tank (read plug in the tank heater on the ground until up to temperature/pressure)

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Yes. You want to minimise the volume of high pressure whatever. The flash boiler system would be a lot more complciated than a big old tank, though.

Well-Known Member

No jumping to conclusions, just asking for reference. You wrote Since I have experience with the first generation of utility scale supercritical cycles, I would like to get current on the “new supercritical steam turbines that vastly reduce turbine size.” Why not provide an answer rather than a glib comment?

BJC

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I have an old copy of a huge Babcock and Wicox steam book. It's my best steam turbine reference.
The fact that it is written for nuclear power plant engineers does not influence the physics involved.